I'm a little lost over what Oerberg is doing with tantum in this sentence...

Nōlī ā mē postulāre ut tantum pecūniae statim solvam.

His note specifically introduces tantum as an adjective, but in that case what is it agreeing with? The only candidate I can see is pecūniae, which can only be the genitive or dative singular or nominative plural, none of which agrees with tantum. And why isn't it (pecūniam) in the accusative anyway, as the object of solvam?

The only thing I can think of is that tantum is agreeing with a implied neuter noun in the accusative on the lines of "sum / pile / lot / heap /bunch" etc, with pecūniae as a singular genitive also hanging off that noun - ie "Don't ask that I should pay you such a big wad of cash immediately!"

David, you are exactly right: tantum is being used here substantively; like many words in Latin, it can function as an adjective or as a noun. We do the same in English: "I can only take so much!" (nominal) ... or ... "Latin pronouns cause me so much trouble!" (adjectival). What's tricky here is that in addition to the nominal use of the neuter adjective, you also have an idiomatic use of the genitive. It is sometimes call partitive genitive - we say "so much money" in English, but in Latin you say "so much (of) money," tantum pecuniae.Just to brace yourself for even more confusion later, the neuter tantum is also commonly used as an adverb in Latin (many Latin adverbs are neuter adjectives). So, depending on context, tantum can be an adjective, or a noun, or an adverb. The HUGE article about it in the Lewis & Short dictonary alerts you to the wide range of uses of this word and also lets you know that is one of the most commonly used Latin words. Here is the dictionary entry:http://athirdway.com/glossa/?s=tantus

You are absolutely right to ask questions! It's always good to ask even when you have just a tiny doubt, and also to look up words in the dictionary even when you are pretty sure what they mean. Sometimes you will find a surprise.Some Latin sayings about how DOUBT is the road to learning! Dubium sapientiae initium.Qui incipit dubitare, incipit sapere.De omnibus dubitandum.Dubitando ad veritatem pervenimus.Ille nihil dubitat qui nullam scientiam habet.Ille nihil dubitat quem nulla scientia ditat.